"Hi, you're live on Math Chat!" The question from the caller sounds pretty tough: "Three salesmen visit a shop. One comes every four days, another every five days, and the third every 10 days. If all of them were at the shop today, how long would you have to wait to see them together again?" But it's not too tough for Frank Morgan, a math professor at Williams College in Williamstown, Mass., and his student costar, Eric Watson. Sitting in front of the camera, the two hosts deftly toss the challenge back to the audience and go on to the next caller. Morgan's half-hour call-in program airs every Sunday night at 6:30 p.m. on Willinet, the local cable community access channel.